1. Field of the Invention
The present invention relates to a controller for injection molding machine. Particularly the present invention relates to a controller characterized in pressure hold control.
2. Description of the Related Art
In injection molding, an injection step for injecting molten resin to fill a mold cavity is carried out, and thereafter a pressure holding step for holding the filled resin at predetermined pressure is executed. Injection speed control is generally performed in the injection step, and pressure control is performed in the pressure holding step.
In the pressure holding step, the screw is caused to move backward by controlling the pressure by means of holding pressure which is lower than the pressure on the resin at the time of completion of the injection step, to perform control so as to obtain the set holding pressure. A significant change in pressure occurs immediately after switching from the speed control in the injection step to the pressure control in the pressure holding step, thus a sink mark, short shot, or other molding failure occurs. For this reason, there have been known a method of suppressing the occurrence of sink marks and the like by limiting the speed of backward movement of the screw in the pressure holding step (see Japanese Patent Application Laid-Open No. 2003-326573 (referred to as “Patent Literature 1” hereinafter)), and a method of controlling the torque of a motor driving the screw on the basis of a limiting value of the speed of backward movement of the screw and the speed of backward movement of the screw, and performing control so that bubbles are not generated inside the resin when switching from the injection step to the pressure holding step (see Japanese Patent Application Laid-Open No. 9-220740 (referred to as “Patent Literature 2” hereinafter)).
When switching from the injection step to the pressure holding step, switching of the screw moving direction from the forward direction (injection direction) in the injection step to the backward direction becomes difficult due to the effect of inertia force of the screw or the like, thereby causing overfilling of resin and deteriorating the quality of molded articles. For this reason, there has been also known a method of performing control so as to prevent such overfilling of resin by generating maximum torque on an injection motor in the braking direction (backward direction) until speed for limiting the holding pressure is reached or for a predetermined period of time, and stopping the forward movement of the screw rapidly, when switching from the injection step to the pressure holding step (see Japanese Patent Application Laid-Open No. 4-329113 (referred to as “Patent Literature 3” hereinafter) and Japanese Patent Application Laid-Open No. 8-281729 (referred to as “Patent Literature 4” hereinafter)).
There has been also known a method in which, in order to avoid the influence of the inertia energy by stopping forward movement of the screw rapidly, low speed at which the inertia energy of the injection screw or the like can be disregarded is set in the injection step when switching to the pressure holding step, so that the injection step where speed control is performed can be shifted to the pressure holding step where pressure control is performed, without being affected by the inertia energy when switching from the injection step to the pressure holding step (see Japanese Patent Application Laid-Open No. 59-224324 (referred to as “Patent Literature 5” hereinafter)).
Also, there has been known a method in which the speed of forward movement of a plunger is reduced at a set acceleration until the pressure inside the cylinder reaches commanded pressure, when the step has shifted from the injection step to the pressure holding step, and thereafter control for holding the commanded pressure is performed (see Japanese Patent Application Laid-Open No. 11-188763 (referred to as “Patent Literature 6” hereinafter)).
According to the methods described in Patent Literatures 3 through 6 above, the amount of forward movement of the screw is reduced when shifting from the injection step to the pressure holding step in accordance with the inertia energy of the screw or the like. Furthermore, in the methods described in Patent Literatures 1 and 2 above, the occurrence of sink marks or bubbles is prevented by limiting the speed of backward movement of the screw when the screw starts to move in a backward direction from a forward direction when the injection step is shifted to the pressure holding step, so that abrupt reduction of the pressure after the screw has started the backward movement can be suppressed. However, the pressure which has dropped immediately after starting to hold the pressure cannot be controlled, thus the object of preventing the occurrence of sink marks or bubbles cannot be achieved sufficiently.
FIG. 8 is a figure showing the relationship among the speed of a screw, torque of an injection motor driving the screw, and the pressure applied to a resin when applying control for limiting the speed of backward movement of the screw after switching the step to the pressure holding step. In FIG. 8, (a) shows the speed of the screw, (b) shows output torque of the injection motor driving the screw, and (c) shows the pressure applied to the resin.
In the injection step, the screw is driven at a set injection speed, as shown in (a) of FIG. 8. In the meantime the resin is filled into a mold cavity, and the pressure applied to the resin (injection pressure) rises, as shown in (c) of FIG. 8. When a switching point (point at time t1) at which the injection step is switched to the pressure holding step is reached, the control performed in the injection step is switched to pressure control, and, as shown in (b) of FIG. 8, the injection motor generates maximum torque in an opposite direction (direction for causing the screw to move backward), trying to stop the forward movement of the screw. It should be noted that the moving speed and torque of the screw in the forward direction are referred to as “positive speed” and “positive torque,” and the moving speed and torque of the screw in the backward direction are referred to as “negative speed” and “negative torque” hereinafter. The screw moves forward by means of the inertia energy even if the injection motor generates maximum negative torque (the methods described in Patent Literatures 3 through 6 reduce the amount of this forward movement), and the pressure applied to the resin increases during the forward movement, as shown in (c) of FIG. 8.
When the speed becomes a negative value from a positive value at time t2 and the screw starts to move backward, the pressure applied to the resin starts to decrease as shown in (c) of FIG. 8. At this moment, when no limits are placed on the speed of the backward movement of the screw, the speed of the backward movement of the screw is large, thus the pressure (holding pressure) indicated with the solid line in (c) of FIG. 8, described with “before limiting the speed”, is applied to the resin, and the pressure decreases abruptly. As in the methods described in Patent Literatures 1 and 2 above, on the other hand, if control is performed with a limiting value provided to the speed of the backward movement of the screw (see (a) of FIG. 8), the speed of the backward movement of the screw is limited, thus the pressure (holding pressure) applied to the resin decreases moderately as shown in the graph indicated with the dashed line in (c) of FIG. 8, described with “after limiting the speed”, so that the pressure can be prevented from being reduced abruptly and the occurrence of sink marks or bubbles can be suppressed.
However, as shown in FIG. 8, since the forward movement of the screw is switched to the backward movement to allow the screw to start moving backward, until the speed of the backward movement reaches a set limiting value of the speed of the backward movement, any kind of control is not performed, thus the pressure applied to the resin decreases drastically. Specifically, between the time t2 and time t3 in FIG. 8, the pressure applied to the resin decreases drastically (see (c) of FIG. 8) and control is not performed on the pressure. Therefore, such drastic decrease of the pressure during the above time becomes a cause of the occurrence of sink marks of bubbles, deteriorating the quality of molded articles.